The present invention relates to swirl combustion, and more particularly to a combustion apparatus and related method that provides high combustion intensity and efficient heat transfer to combustion chamber walls, while producing exhaust gases with a low concentration of nitrogen oxide, commonly known as NO.sub.x.
Swirling flows have been used in combustion chambers to improve flame stability and combustion by generating toroidal recirculation zones within a combustion chamber, and by reducing combustion time by producing rapid mixing of fuel and air within the chamber, particularly at the boundaries of the recirculation zones. Two important types of swirling combustors are swirl burners and cyclone combustion chambers.
Cyclone combustion chambers have been used to produce a cyclone of turbulent gases within a combustion chamber for combusting various solid materials, including poor quality coal and vegetable refuse. Such combustors are disclosed in "Combustion in Swirling Flows: A Review", N. Syred and J. M. Beer, Combustion and Flame, Vol. 23, pp. 143-201 (1974); in U.S. Pat. Nos. 4,457,289, and 044,735 filed May 1, 1987 to Korenberg, all of which are incorporated herein by reference.
Although known adiabatic cyclone combustors provide high specific heat release, such known combustors have the disadvantage that combustion temperature is high and NO.sub.x emissions are high. In conventional cyclone combustors, combustion is unstable at low capacity burning and high turndown ratios are not possible in non-adiabatic combustors.
The turndown ratio of a combustion apparatus in a boiler is defined as the ratio of maximum load to minimum load; and measures the ability of the boiler to operate over the extremes of its load ranges. A high turndown ratio allows for a wide range in the level of steam energy generation at a particular time. A wide range of steam energy generation is important to allow the boiler to most efficiently respond to varying steam energy demands. As a result, it is desirable that improvements in swirling efficiency and heat transfer in the boiler combustion chamber and combustion chamber outlet not decrease the turndown ratio of the boiler.
Stable combustion can be achieved by not cooling the walls of a cyclone combustion chamber in the portion of the chamber into which air and fuel are injected for combustion, as is disclosed in U.S. patent application Ser. No. 928,096, filed Nov. 7, 1986 to Korenberg et al. which is incorporated herein by reference. High wall temperatures near the chamber's fuel and air entrance enable a high turndown ratio to be achieved. For example, by incorporating uncooled refractory lined walls at the air and fuel entrance to the combustion chamber, the turndown ratio of maximum to minimum load can be increased from 4:1 up to and higher than 10:1. With such an arrangement, excess air over that required as a combustion reactant, can be decreased from 25-30% to about 5% and kept constant at about 5% over the turndown ratio of 10:1. In addition, the flame temperature can be decreased from about 3000.degree. F. to about 2000.degree. F. for conventional fire tube boilers. By lowering the excess air and by lowering the flame temperature, NO.sub.x emission concentrations are lowered in the flue exhaust.
With pollution control requirements becoming constantly more stringent, it is necessary to decrease NO.sub.x emissions even further than is achieved with the combustion apparatus described above, without increasing the cost of the combustion equipment.
In the prior art cyclone combustion chamber shown in FIG. 1, as it is described in the previously identified publication by Syred & Beer, air and fuel are injected tangentially through an air inlet 16 into a large cylindrical combustion chamber 17 in which the air is combined with burning fuel. Hot combustion gases circulate and recirculate in combustion chamber 17, and then exhaust through a centrally located exit throat 18 in one end of combustion chamber 17. Combustion occurs primarily inside the cyclone chamber, and is aided by large internal reverse flow zones represented generally by the direction of the arrows within chamber 17, which provide a long residence time for the fuel/air mixture. In contrast to a swirl burner, which usually has one central toroidal circulation zone, the cyclonic combustor often has up to 3 concentric toroidal recirculation zones. The long residence time and large number of reverse flow zones result in a high specific heat release from combustion in the chamber.
A summary of the general aerodynamics of such cyclones can be seen in FIG. 1. Five characteristics annular zones are distinguishable. There are two main downward flows, rotating coaxially, which carry the main mass of gas, namely the wall flow represented by arrows 22, and axial flow in the central area represented by arrows 24. In these flows, the maxima of tangential velocity W and axial velocity U are combined. Flows 22 and 24 are divided by a peripheral intermediate zone, occupied by the rising turbulent vortices branching from flows 22 and 24, and forming as a result the reverse stream 26. In zone 28, the profiles of W are dip, U is reversed, and hence the tangential and axial velocity profiles are saddle-like in form, varying over the cyclone height. In central zone represented by arrows 30, two slightly twisted axial flows move opposite to each other, a direct flow from the top, and a reverse flow from the exits as shown by the direction of the arrows.
A large portion of the gas, without reaching the cyclone exit, develops an axial velocity and leaves the top, forming flow 24. However, some of the gas in this top boundary layer is carried over to the cyclone exit and forms the weak descending flow represented by arrows 30. This flow rapidly decays toward zero within one chamber diameter.
It is worth noting that the maximum intensity of turbulence occurs at and around the peak of tangential velocity in region 24. The intensity of turbulence is approximately 5 times lower near the outer wall 20. If the exit throat is removed (the cyclone chamber within wall 20 is then similar to the swirl generator shown in FIG. 2), the root mean square values of the velocity fluctuations (U'.sup.2 and W'.sup.2) are 1.5 times higher near the walls, and in the main flow (region 203) 3 to 5 times lower than in the cyclone chamber which has a throat.
It is interesting to compare the efficiency of swirl generation in the cyclone combustor and swirl combustor. With the swirl burners, the efficiency of swirl generation is based upon the swirl generated at the exit throat. Usually, as most of the volume recirculation zone, high levels of turbulence and hence mixing occur past the exit throat, it must be expected that increasing the level of efficiency (for a given swirl number), will increase the recirculating mass flow, level of turbulence, and mixing rate. As internal reverse flows are only infrequently formed, little dissipation of swirl energy occurs inside a swirl burner and hence, efficiencies of swirl generation as high as 70-80% can be obtained.
Different criteria apply to cyclone combustion chambers. It has been shown that efficiencies of swirl generation in cyclone combustion chambers are typically 8-15%. This figure was obtained by integration of measured tangential velocity profiles inside the cyclone chamber and comparison with the input energy, and hence include energy dissipation due to the formation of internal reverse flows and high levels of turbulence inside the cyclone chamber.
It can well be argued for cyclone combustion chambers that as long as input losses are minimized, a low to intermediate value of efficiency is beneficial as the energy balance has then been altered toward the production of large internal reverse flows and high levels of turbulent mixing.
Thus, turbulence and recirculation in cyclone combustion chambers have the effect of reducing swirling efficiency of the chamber because of the large amount of turbulence within the chamber, especially in the area of the exit throat. Further, in cyclone combustion chambers, the velocity of air flow along the wall of the combustion chamber is significantly decreased by turbulence and recirculation, especially near the end of the chamber where the exit throat is located. With decreased tangential velocities near the chamber wall, heat transfer to the chamber wall is reduced so that the chamber, if cooled, is not cooled as effectively as would be possible if air velocities were greater near the combustion chamber wall. Reduced cooling efficiency results in higher emissions of NOx for a chamber of the same volume due to the higher combustion temperature.
In a swirl burner, the swirling flow exhausts into a furnace or cavity and combustion occurs in and just outside the burner exit. Two principal modes of swirl generators are in common use: (a) guide vanes in axial tubes and (b) tangential entry of the fluid stream, or part of it, into a cylindrical duct. Despite the differences in configuration, there are many similarities in the flow patterns produced by different types of swirl generators.
A swirl number is a measure of the angular momentum of a swirling fluid in comparison to the linear momentum of the fluid. A higher swirl number is indicative of greater angular momentum and swirling. Swirl numbers of typical swirl burners are usually in the range of 0.6 to 2.5. A large toroidal recirculation zone is formed in the exit, occupying up to 75% of the exit diameter, with up to 80% of the initial flow being recirculated, the swirl number being 2.2. The tangential velocity distribution is of Rankine form (i.e., free/forced vortex) inside the swirler, decaying into a forced vortex distribution at the exit plane.
It is interesting to note that generally for swirl numbers less than 2.4, confinement increases the central recirculated mass flow, while swirl numbers greater than about 1.6 the outer region of circulation disappears. This occurs because, upon leaving the swirl generator, the swirling flow sticks immediately to the walls of the confinement while further downstream, complex recirculation patterns similar to those of cyclone combustion chambers develop.
In a swirl burner, as shown in FIG. 2, a swirling flow of air is tangentially introduced by an air inlet 32 into an air plenum 34 surrounding a swirl chamber 36 into which fuel is axially introduced by fuel inlet 38. Solid end plates 40 seal the ends of air plenum 34 so air injected into air plenum 34 is forced through slits 42 in the wall of swirl chamber 36. Combustion takes place primarily just outside a burner exit 44 with some combustion also occurring within the burner swirl chamber 36.
In swirl burners, large toroidal recirculation zones are generally formed in exit 44 and occupy up to 75 percent of the exit diameter with up to 80 percent of the initial flow being recirculated. Swirl numbers for swirl burners are usually in the range of 0.6-2.5.
As noted above, because swirl burners have an open unrestricted outlet, little dissipation of swirl energy occurs inside the burner and hence, high swirling efficiencies of 70-80% can be obtained, as compared to typical swirling efficiencies of 8-15% for cyclone combustion chambers. However, turbulence in the swirl burner outlet and downstream of the outlet results in dissipation of swirl energy and reduced swirling in the burner outlet and downstream of the outlet. Lower tangential velocities of combustion gases in the outlet and downstream of the outlet, where much of the combustion takes place, makes transfer of combustion heat less efficient. Inefficient heat transfer increases combustion temperatures which results in increased exhaust concentrations of pollutants such as NO.sub.x.